As electronic devices become smaller, the requirements for precise electrical connection at extremely fine pitch continue to increase. As an example, semiconductors, such as integrated circuits, are formed on wafers that are then cut into dice or chips that individually may be mounted on substrates. Typically, the substrate has fine electrically conductive circuit lines, and electrical and thermal contact must be made between the substrate and chip. As electronic appliances, such as computers, tape players, televisions, telephones, and other appliances become smaller, thinner, and more portable, the size requirements for semiconductors and the means for providing electrical connection between semiconductors and substrates, or between flexible circuits and rigid printed circuits, become increasingly demanding.
One method for providing electrical conductivity between two electrical elements, is through the use of a Z-axis conductive sheet material, such as a Z-axis adhesive. Whether the sheet material is an elastomer or an adhesive, the continuing challenge is to keep pace with the miniaturization in the electronics industry. Z-axis conductivity can be achieved through a number of means, including dispersing conductive particles throughout a binder matrix. Where electrical connection on a very fine pitch is required, the conductive elements may be placed only where the electrodes are located, typically requiring indexing the conductive sheet to the electrodes, or the conductive elements may be placed at such close spacing, relative to the spacing of the electrodes, that indexing is not required. U.S. Pat. No. 5,087,494, (Calhoun et al) is an example of an electrically conductive adhesive tape having conductive particles placed at precise locations, on a fine pitch. The Calhoun et al '494 patent also discusses a number of available options for electrically conductive adhesive tapes.
U.S. Pat. Nos. 4,008,300 (Ponn) and 3,680,037 (Nellis, et al.), teach a dielectric sheet material having a plurality of compressible resilient conductive plugs that extend between the faces of the sheet. The sheet can be placed between circuits to make electrical connection there between. The conductive plugs of Ponn and Nellis are dispersions of conductive particles in a binder material.
Other patents teach orienting magnetic particles dispersed in a binder by applying a magnetic field, e.g., U.S. Pat. Nos. 4,448,837 (Ikade, et al.); 4,546,037 (King); 4,548,862 (Hartman); 4,644,101 (Jin, et al.); and 4,838,347 (Dentinni). The distribution of the particles after orientation and curing is sufficiently uniform to be functional for certain applications, but is insufficient for other applications. If the number of particles used in these articles were to be increased in an attempt to reach smaller spacings for finer pitch connections, agglomeration would likely occur thereby causing shorting. Accordingly, there is a need for a fine pitch means of providing electrical interconnection between two surfaces in a precise manner, at an extremely fine pitch.
Another deficiency in prior attempts to make electrical interconnection utilizing electromagnetic particles is that the bonding surface is not conducive to ready interconnection between two conductive surfaces.